1. Field of the Invention
This invention relates to the field of semiconductor devices. More particularly, the invention relates to a method and apparatus for resolving conflicts between phase shifted structures and non-phase shifted structures during the definition of masks to be used in optical lithography processes for manufacturing integrated circuit devices.
2. Description of the Related Art
Semiconductor devices continue to be produced at reduced sizes as optical lithography processes have evolved. Techniques such as phase shifting have been developed to assist in the production of subwavelength features on the integrated circuits (IC) using optical lithography processes. Subwavelength features are features that are smaller than the wavelength of light used to create circuit patterns in the silicon. More generally, phase shifting can be used to create features smaller than a minimum realizable dimension for the given process.
Through the use of phase shifting masks, such subwavelength features can be efficiently produced. (Note, that the term xe2x80x9cmaskxe2x80x9d as used in this specification is meant to include the term xe2x80x9creticle.xe2x80x9d) One approach to producing a phase shifting mask (PSM) is to use destructive light interference caused by placing two, out of phase, light transmissive areas in close proximity in order to create an unexposed region on a photoresist layer of an IC. If that unexposed area is then protected from exposure when a binary mask is used to expose the remaining field (thus causing definition of the remaining structure), the resultant IC will include subwavelength features created by the PSM.
One approach to preparing an IC for production using PSMs is for one or more features of the IC to be identified for production using PSMs. For example, a designer might identify one or more particular features for production using the PSM, e.g. to define the identified gates (or other features) at subwavelength sizes.
A portion of a design layout 100 for a layer in an IC is shown in FIG. 1. Several distinct portions of the design layout are identified, particularly a field polysilicon 104, a gate 102 and a structure 106. In this example, the gate 102 is identified as xe2x80x9ccriticalxe2x80x9d, e.g. specified for production using a PSM.
A phase shifting mask 200 for defining the gate 102 is shown in FIG. 2. Light transmissive region 202 and light transmission region 204 are out of phase with one another, e.g. light through one is at phase 0 and the light through the other at phase xcfx80. These light transmissive regions are sometimes referred to both individually and collectively as phase shifters (the meaning will be apparent from usage). Additionally, the light transmissive regions are sometimes referred to as phase shifting areas. Also shown on FIG. 2 is an outline 206 of where the structure 106 is relative to the openings for the phase shifters. Particularly, the outline 206 is overlapped by the light transmissive region 204.
Thus, FIG. 2 generally illustrates one example of the class of problems to be addressed. In particular, it is generally preferable to make the phase shifters (e.g. the light transmissive region 202 and the light transmissive region 204) relatively wide compared to the wavelength of the light (xcex). For example, some PSM processes attempt to make the total width of the phase shifters and the protective area between them approximately 3xcex. However, it is unacceptable to allow the phase shifters to directly overlap areas where there are non-phase shifted structures (e.g. overlap area 208).
Similarly, because of optical effects, e.g. light traveling under the phase shift mask, due to mask misalignment between the PSM and the binary mask, etc., some non-phase shifted structures in close proximity to where phase shifters are placed on the PSM may become exposed at the time the PSM 200 is used.
Accordingly, what is needed is a method and apparatus for resolving conflicts resulting from placement of phase shifters in close proximity to structures being produced other than by phase shifting. Additionally, both a PSM and a binary mask that can produce ICs with subwavelength structures that are in close proximity to at or above wavelength structure are needed.
Methods and apparatuses for preparing layouts and masks that use phase shifting to enable production of subwavelength features on an integrated circuit in close proximity to other structures are described. Because light can bend around the edges of phase shifting areas in the phase shifting mask, optically proximatexe2x80x94as well as overlappingxe2x80x94structures can become exposed.
In one embodiment, a single edge of the phase shifter is moved away from a corresponding edge in the conflicting structure so that the optical proximity problem is removed. The distance the edge is moved will be based on the wavelength of light (xcex) used to produce the IC as well as other information about the optical lithography process. For example for one xcex=248 nm process, a distance of approximately 50 nm is used. Thus if the right edge of a phase shifter is less than 50 nm from an edge of a structure (or overlaps the structure) that will be defined other than be phase shifting, the right edge is moved further from the structure""s edge to be at least 50 nm away.
Another embodiment optically corrects the shape of the phase shifters in proximity to a conflicting structure. In one embodiment, the edge(s) of the shifter closest to the structure are reshaped to follow the edges of the conflicting structure at a predetermined distance from the structure. For example, in one embodiment 0.2xcex is used as a minimum distance. For example, if only the bottom half of the right edge is within 0.2xcex of the conflicting structure, then primarily that bottom half is moved further from the edge to be at least 0.2xcex away. If the phase shifter started with a rectangular shape, this embodiment can result in non-rectangular shifter shapes.
One embodiment selects from several strategies for resolving conflicts between phase shifters used to define features and proximate structures. For example, one strategy is to change the layout to move structures so that they are no longer in close proximity to the features selected for definition using phase shifting masks. However, a designer might disable this strategy so that it was not available for use in resolving conflicts found in a particular design. The designer might do this if they prefer not to allow their design to be altered, e.g. because only certain cell libraries are to be used, because they do not want to re-verify the design, etc. In this embodiment, the modified layout may, or may not, have the same netlist electrical characteristics as the original layout.
One embodiment adds additional phase shifters to define the conflicting structures. In this approach, a conflicting structure that was not initially designated for definition using phase shifting is marked for definition using phase shifting. This approach may introduce conflicts with previously placed phase shifters as well as other structures. However, in many cases it will allow maximal use of phase shifters to define the features marked for production using phase shifting while maintaining preferred phase shifter sizes.
When this approach is used the binary trim mask used in conjunction with the phase shifting mask will have to be modified as well. This approach may also improve IC yield and IC performance.
In other embodiments, the conflicting structures are sized up, or biased.
In one embodiment, different areas of a layer and/or an integrated circuit can be treated using different techniques. In another embodiment, all areas of a layer and/or an integrated circuit are treated using a single technique.
Resulting integrated circuits can include a greater number of subwavelength features even in areas that are in close proximity to structures that were not initially identified for production using a phase shifting mask.